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Comparative Neuroanatomy of Mollusks and Nemerteans in the Context of Deep Metazoan Phylogeny

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Comparative Neuroanatomy of Mollusks and Nemerteans in the Context of Deep Metazoan Phylogeny Comparative Neuroanatomy of Mollusks and Nemerteans in the Context of Deep Metazoan Phylogeny Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH Aachen University zur Erlangung des akademischen Grades einer Doktorin der Naturwissenschaften genehmigte Dissertation vorgelegt von Diplom-Biologin Simone Faller aus Frankfurt am Main Berichter: Privatdozent Dr. Rudolf Loesel Universitätsprofessor Dr. Peter Bräunig Tag der mündlichen Prüfung: 09. März 2012 Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar. Contents 1 General Introduction 1 Deep Metazoan Phylogeny 1 Neurophylogeny 2 Mollusca 5 Nemertea 6 Aim of the thesis 7 2 Neuroanatomy of Minor Mollusca 9 Introduction 9 Material and Methods 10 Results 12 Caudofoveata 12 Scutopus ventrolineatus 12 Falcidens crossotus 16 Solenogastres 16 Dorymenia sarsii 16 Polyplacophora 20 Lepidochitona cinerea 20 Acanthochitona crinita 20 Scaphopoda 22 Antalis entalis 22 Entalina quinquangularis 24 Discussion 25 Structure of the brain and nerve cords 25 Caudofoveata 25 Solenogastres 26 Polyplacophora 27 Scaphopoda 27 i CONTENTS Evolutionary considerations 28 Relationship among non-conchiferan molluscan taxa 28 Position of the Scaphopoda within Conchifera 29 Position of Mollusca within Protostomia 30 3 Neuroanatomy of Nemertea 33 Introduction 33 Material and Methods 34 Results 35 Brain 35 Cerebral organ 38 Nerve cords and peripheral nervous system 38 Discussion 38 Peripheral nervous system 40 Central nervous system 40 In search for the urbilaterian brain 42 4 General Discussion 45 Evolution of higher brain centers 46 Neuroanatomical glossary and data matrix – Essential steps toward a cladistic analysis of neuroanatomical data 49 5 Summary 53 6 Zusammenfassung 57 7 References 61 Danksagung 75 Lebenslauf 79 ii iii 1 General Introduction Deep Metazoan Phylogeny The concept of phylogeny follows directly from the theory of evolution as published by Charles Darwin in The origin of species (1859). According to this theory contemporary species share a common history through their ancestry. In the decades following 1859 first attempts for understanding the evolutionary history and reconstructing the phylogenetic relationships among animals were based on morphological comparisons. This approach lasted until the late 20th century when molecular methods advent and changed the traditional view on the animal tree of life (Fig. 1.1). The so-called “new animal phylogeny” (Adoutte et al. 2000) was initially based on the analysis of the nuclear small ribosomal subunit (18S) gene and rearranged the Bilateria into three clades: Deuterostomia, Lophotrochozoa, and Ecdysozoa (Fig. 1.2). The clade Lophotrochozoa, comprising annelids, mollusks, and the lophophorate phyla, was first introduced by Halanych et al. (1995). Shortly after, Aguinaldo et al. (1997) proposed the clade Ecdysozoa containing arthropods and other molting animals. The most prominent discrepancy resulting from this classification is the relative position of annelids and arthropods. Based on morphological properties, annelids and arthropods were grouped together in a single clade called Articulata. In contrast, molecular studies place annelids and arthropods into the different superphyla Lophotrochozoa and Ecdysozoa. Consequently, the “new animal phylogeny” was disputed by many morphologists (Wägele et al. 1999; Wägele & Misof 2001; Scholtz 2002). In addition, several multigene analyses failed to find support for the “new animal phylogeny” (Blair et al. 2002; Dopazo et al. 2004; Rogozin et al. 2007). However, recent phylogenomic studies using a multitude of species have confirmed the “new animal phylogeny” (Philippe et al. 2005; Helmkampf et al. 2008; Dunn et al. 2008). Despite this corroboration for grouping protostomes into Lophotrochozoa and Ecdysozoa, the relationships within these two superphyla vary strongly between different molecular analyses. Thus, even 150 years after The origin of species (Darwin 1859) the phylogenetic relationships of most major animal groups are still controversial. Therefore morphological characters are still needed as an independent approach to verify the molecular data. 1 GENERAL INTRODUCTION Figure 1.1 – The traditional view of animal phylogeny. The phylogenetic tree illustrates major concepts that are based on the analysis of morphological data. From Halanych (2004). Neurophylogeny In this approach one structure promising a multitude of morphological characters is the nervous system. The relevance of neuroanatomical characters for the inference of phylogenetic relationships was already investigated in the beginning of the 20th century by Nils Holmgren (1916) and his pupil Bertil Hanström (1928). They were among the first to characterize the internal brain anatomy of numerous invertebrate taxa, especially of arthropods, and thus added fundamental knowledge in arthropod evolution. However, in some extends their descriptions were rather superficial and the number and quality of original data presented unsatisfactory. Due to methodological advancements like immunohistochemistry and confocal laser scanning microscopy the field of comparative neuroanatomy has regained new impulses during the past decade and is now often referred to as “neurophylogeny” (Paul 1989; Harzsch 2002; Harzsch 2006). In addition to the technical 2 GENERAL INTRODUCTION Figure 1.2 – The “new animal phylogeny”. The phylogenetic tree is based on molecular data and illustrates the classification of Bilateria into Deuterostomia, Lophotrochozoa, and Ecdysozoa. From Halanych (2004). progress, the methodological background for this discipline mainly relies on the foundation laid out by Kutsch and Breidbach (1994) who established criteria for comparing neuroanatomical characters between different species of arthropods. Based on these criteria, the nervous system has already been used extensively and as well successfully for the inference of phylogenetic relationships within the arthropods (Strausfeld 1998; Harzsch & Waloszek 2000; Loesel et al. 2002; Strausfeld et al. 2006a; Strausfeld & Andrew 2011). In addition, neuroanatomical data can also be utilized in a second way. By mapping neuroanatomical characters on trees that are generally accepted the evolution of particular structures of the nervous system can be retraced. Arthropoda is the largest phylum of invertebrates and therefore it is not surprising that the amount of literature on the brain anatomy of this group is vast, first and foremost that of insects. In addition, the brain of arthropods provides a wealth of morphological features. 3 GENERAL INTRODUCTION Figure 1.3 – Architecture of an insect (a) and an annelid (b) brain. a Schematic diagram showing the major neuropils of the insect brain. Modified from Strausfeld et al. (1998). b Three-dimensional surface reconstruction superimposed upon an autofluorescence image demonstrating that the annelid brain is composed of similar neuropils. From Heuer and Loesel (2009). ca calyx; cc central complex; ey eye; gc globuli cells; lo lobe; mb mushroom body; og olfactory glomeruli; pd peduncle. Scale bar: b = 200 µm. Figure 1.3a displays the major neuropils of the insect brain: the paired mushroom bodies, the central complex (green), and the olfactory glomeruli (yellow). The most prominent of these neuropils are the mushroom bodies built by the ramifications of the so-called globuli cells that for historical reasons in insects are called Kenyon cells. The cell bodies of thousands of these neurons form a dense cluster that surrounds the input region of the mushroom bodies, the so-called calyces. The mushroom bodies receive multimodal sensory input and play a role in associative learning and memory formation (Heisenberg 2003; Campbell & Turner 2010). The mushroom bodies as well as the remaining neuropils are highly conserved and present basically in all arthropod groups, even in onychophorans (Strausfeld et al. 2006a; Strausfeld et al. 2006b). In comparison to the vast amount of neuroanatomical studies on arthropods, analyses focusing on neuroanatomical characters in non-arthropod protostome phyla are rare. However, a recent neuroanatomical study on annelids (Heuer 2010) demonstrates that the brain of polychaete annelids is composed of similar neuropils as the arthropod brain (cf. Fig. 1.3a, b). Moreover, the most prominent neuropil of the arthropod brain, the mushroom bodies are built just in the same way in polychaete annelids, implying a possible homology of arthropod and annelid mushroom bodies. Recently, the morphological-derived homology assumption has been corroborated by molecular fingerprint studies, providing as well strong evidence for a homology of insect and annelid mushroom bodies (Tomer et al. 2010). In the light of the “new animal phylogeny” the homology of arthropod and annelid mushroom bodies implies that these structures have to be a plesiomorphic character trait of all protostomes. Since comparably well-developed mushroom bodies have not yet been identified in any other protostome clade, the homology of arthropod and annelid mushroom bodies requires a secondary reduction of these neuropils in almost all protostome taxa. 4 GENERAL INTRODUCTION Figure 1.4 – Neuroarchitecture of a non-conchiferan mollusk (a) and a nemertean (b) representative. a Schematic diagram showing the major components of the anterior nervous system of Syngenoherpia intergenerica (Solenogastres, Mollusca). Modified from Salvini-Plawen (1972). b Schematic diagram showing the major components of the anterior
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